Many varieties of self-tapping sheet metal screws having ribs, grooves or teeth around their shanks and heads to increase the resistance of the screws to removal once they are engaged for fastening metal sheets, are well known and are shown in the prior art. However, there is no known reference disclosing or teaching an unsymmetrical or incomplete circumferential series of ribs around a shank adjacent the cupped-shape head of the screw as discovered by applicants.